I don't know if there is any way to "demo" a stem on your bike unless you've got an exceptionally cool LBS (they exist!) or a cooperative friend with a large stash of spare parts, but you're really going to have to try the stem on your bike to know which one's right for you.

I can tell you this; I changed to a 50mm Thomson stem from the stock 75mm one that came on my Stumpy Comp FSR 29er, and it might be the best thing I've ever done for the bike. It's easier to get the front end up and over stuff, it does drops better... All without making the steering twitchy or weird.

I run an 80MM stem on my XL Anthem X 29er. I bought a brand new take off Giant stem on eBay for $12.00 It worked out well because I found the same one that came on the bike only much shorter. any shorter for me might be too twitchy but the XL/80MM combo works for me...but I am 6'3"

I looked around and finally bought one of these Chinese stems. I bought the 70 mm one from the same seller, $28. I was bit concerned about a cheap stem but if you think about it the torque at the base of the stem is reduced by 36% going from 110 mm to 70 mm. T = Fxd.
It is lite and the color was almost identical to my longer Specialized stem. My original thought was the same as the last poster. Try it out and buy a more expensive one if I liked it. Can't see spending the money now. I am a pretty conservative trail rider so it's hard to imagine I can break one of these.

I ride a 29er Spec Camber. I have noticed my front end seemed "twitchy", kind of hard to keep it on a line. I decided to put my original 110 mm stem back on in place of the 70 mm I had switched to. The bike immediately felt like it tracked better.

Just to be clear an upgrade is changing one or more component(s) to something else that is lighter/stronger/better. Changing a stem to a shorter size is dialing/tweaking the fit and handling characteristics of a bike. Carry on.